Lynn Swanson, age 12, of North St. Paul, Minnesota, for her question

 What causes a timber line?

There must be a reason why forests grow so far and no farther up the slopes of a high mountain. There must be a reason why the northern trees halt at the doorstep of the Arctic tundra. These rigid demarcations are called timber lines and they are set by rigid rules of nature.

A tall mountain peak may be anywhere from 2 1/2 to 5 1/2 miles above the level of the sea. The weather, the atmosphere and other conditions change with every step up in the altitude. And every set of conditions favors its own plant and animal communities. As we ,journey from the base of the mountain to its summit, we traverse across three, four or even five very different ecology zones. On an African safari, we may begin our climb through a stretch of grassy savanna to reach rolling foothills clothed in dense, green rain forests. Above the jungle zone, we may hack a trail through a tangle of giant bamboo. Above this is a high, dry zone of prairie grasses and clusters of small trees.

At 12,000 feet also, we reach the timber line and leave the last of the trees behind us. We are in the alpine zone where harsh conditions permit only patient, ground hugging plants and the animals that feed upon them. On mountains deep in the steamy tropics, we may not reach the treeless timber line before climbing to 13,000 or 14,000 feet above sea level. In the Rockies and Sierra Nevada ranges, the timber line is 10,000 to 12,000 feet high. In the White Mountains of New England, it is 5,000 feet and on the slopes of Alaska we leave the trees behind at heights of 1,000 to 3,000 feet.

These figures suggest that geography has something to do with the timber line, or at least with its location. This is true. The air becomes colder with altitude and the bitter winds blow stronger, the weather may be drier and the summer growing season is shorter. Taking the world average, the bleak treeless zone occurs at about two miles above sea level. In the tropics, the changes are more gradual and trees can grow at a higher level. In the temperate geographic zones, the timber line is lower and it finally vanishes at sea level in the polar regions. Plants need a fair quota of moisture and warm sunshine. Trees, the biggest plants, need the most. They struggle up the mountain slopes as high as they can, but as conditions grow worse, only the hardy spruces and firs can push on. At last they, too, are left behind At the timber line. Above this point, the weather is too cold and the summer too short for them. The winds that have grown fiercer with altitude bend anti bash down their brave trunks. What's more, the blustering blizzards tend to strip away the meager soil and trees are unable to anchor their deep roots in the ground. These are the main factors that cause even the sturdiest trees to stagger to a halt at the timber line.

From the ground, the evergreen forests seem to come to a sudden halt at the tim¬berline. But as a rule, a few stragglers try to struggle up into the forbidden region above. They strive to survive in the Krummholz zone, the region of crooked wood. Their gnarled trunks are bent and twisted, their branches forced to grow lopsided by the fierce prevailing winds. The tall and supple limber pine of the valleys may grow here, but with its branches flat on the ground. The pliable snow willow spreads its boughs no higher than a few inches, catkins and all.